1. Field of the Disclosure
The present disclosure is related to both logging-while-drilling (LWD) and wireline-logging (WL). In particular, the present disclosure is an apparatus and a method for imaging electrical properties surrounding a borehole having a non-conductive mud therein during or after the drilling.
2. Background of the Art
Electrical logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, inject electrodes (current source or voltage source) are used in conjunction with receiver electrode(s) to measure either voltage difference between two electrodes or electric current that flows through a receiver electrode, earth formation to a return electrode in the tool. In the second category, an antenna (coils) with a current source within the measuring instrument induces eddy current within the earth formation. The induced current generates signals that can be detected using either the same antenna or a separate receiver antenna. The present disclosure belongs to the second category.
Techniques for investigating the earth formation with arrays of induction coils have been proposed. U.S. Pat. No. 6,957,708 to Chemali et al., assigned to Baker Hughes Incorporated, is a method and apparatus for determining an electrical property of a formation surrounding a borehole with a measuring-while drilling device. An antenna configuration is disposed along the outer face of an extendable stabilizer joined to a rotatable drill collar. Typically, two receivers and two transmitters are space along the longitudinal axis of the drill collar, with the two receivers spaced symmetrically in between the two transmitters. Alternatively, receivers and transmitters can be spaced along the circumference, possibly over multiple stabilizers.
U.S. Pat. No. 4,780,678 to Kleinberg et al., assigned to Schlumberger Technology Corporation, provides a wall-engaging apparatus for microinductively investigating a characteristic of an earth formation traversed by a borehole. In one aspect, an antenna set includes first, second and third antenna elements. The second and third elements, being structurally identical but differentially coupled, are positioned in electromagnetic symmetry about the first antenna element. Either the first antenna element or the differentially coupled second and third antenna elements may be energized by suitable circuits, while the other is coupled to circuits for receiving signals indicative of a set characteristic. As noted in U.S. Pat. No. 7,073,609 to Tabanou et al., having common ownership with Kleinberg and also assigned to Schlumberger Technology Corporation, the apparatus of Kleinberg et al. uses a differential induction coil system that responds primarily to contrast in resistivity between adjacent layers. As such, this tool has the capability to measure formation dip in oil-based mud, but it has no imaging capability.
U.S. Pat. No. 4,980,642 to Rodney is directed to a system, apparatus and methods for determining influx of formation fluids into a well borehole during drilling. The methods of Rodney include determining the dielectric constant and/or conductivity of the fluid in the annulus about a drill pipe in a well borehole, comparing the determined dielectric constant and/or conductivity to a reference value and inferring that influx has occurred when the difference between the dielectric constant and/or conductivity and the reference value is greater than a predetermined limit. The dielectric constant and/or conductivity is determined by transmitting electromagnetic energy between about 10 KHz and about 10 GHz through the fluid in the annulus about the drill pipe from a transmitter located in the drill pipe section, receiving a portion of that energy which has traveled through the fluid but not through the surrounding formation in a receiver located in the drill pipe, and determining the dielectric constant and/or conductivity as a function of at least one of the phase and the amplitude of the received energy.
Non-conductive mud is commonly used in wellbore drilling and its presence in the wellbore presents difficulties for wellbore imaging. The present disclosure addresses these issues.